Question 9.
What is primary productivity? Give brief description of factors that affect primary productivity.
Answer:
Primary productivity is the amount of biomass or organic matter produced per unit area over a specific time period by plants during photosynthesis. It is usually expressed in terms of weight (g/m²/yr) or energy (kcal/m²/yr).

There are two types of primary productivity:

1. Gross Primary Productivity (GPP): The total rate of production of organic matter during photosynthesis.
2. Net Primary Productivity (NPP): The organic matter remaining after the plants use some energy for respiration.

Factors Affecting Primary Productivity:

1. Solar Radiation: Maximum sunlight is received in the tropics, leading to higher photosynthesis and NPP (>20 t ha⁻¹ yr⁻¹) compared to temperate forests (~8 t ha⁻¹ yr⁻¹).
2. Temperature: Warm tropical climates favor higher productivity, while cooler temperate regions have lower productivity.
3. Moisture: Adequate rainfall and humidity increase ecosystem productivity.
4. Nutrients: Essential minerals support producer growth. Nutrient-poor soils, like deserts, result in lower productivity.
5. Photosynthetic Efficiency of Producers: C4 plants have higher productivity than C3 plants due to more efficient photosynthesis.

Question 10.
Define decomposition and describe the processes and products of decomposition.
Answer:
Decomposition is the breakdown of dead organic matter and waste into simpler substances. It is a complex, enzymatic process that gradually degrades detritus, making nutrients available for plants and other organisms in the ecosystem.

The process of decomposition involves three main steps:

1. Fragmentation of Detritus: Large organic matter is broken into smaller pieces by detritivores, which increases the surface area for microbial action.
2. Leaching: Water-soluble inorganic nutrients are washed out from dead organic matter into the soil.
3. Catabolism (Humification and Mineralization):

• Humification: Partial decomposition produces humus, a stable organic material that enriches the soil.
• Mineralization: Complete decomposition converts organic matter into inorganic substances such as CO₂, H₂O, and essential nutrients like nitrates and phosphates.

Products of Decomposition:

• Humus: Improves soil fertility and water retention.
• Inorganic Nutrients: Nitrogen, phosphorus, potassium, and other minerals available for plants.
• Gases: CO₂ and H₂O released during decomposition.

Question 11.
Give an account of energy flow in an ecosystem.
Answer:

Energy Flow:
Energy flow is the passage of energy through various trophic levels of an ecosystem. Energy does not circulate; instead, it flows unidirectionally from producers to consumers and finally to decomposers.

1. Capture of Energy:

• Producers capture 1–5% of solar radiation under favorable conditions and use it for photosynthesis, forming gross primary productivity (GPP).
• About 20% of GPP is used by producers for their own life processes.
• The remaining energy, called net primary productivity (NPP), is available to herbivores, approximately 8–4% of incident radiation.

2. Transfer to Herbivores (Primary Consumers):

• Herbivores consume a part of the NPP. The remaining plant matter becomes detritus, which provides energy to decomposers.
• About 30% of ingested energy is used by herbivores for life activities and lost as heat.
• Only ~10% of producer energy is converted into herbivore biomass.
• This transfer of 10% of biomass energy from one trophic level to the next is known as the 10% law.

3. Transfer to Primary Carnivores (Secondary Consumers):

• Primary carnivores feed on herbivores.
• Around 60% of assimilated energy is used in metabolic activities and lost as heat.
• The remaining energy is transferred to secondary carnivores.

4. Transfer to Higher-Level Carnivores:

• 10% of biomass energy from the primary carnivores becomes part of secondary carnivore biomass.
• Similarly, if there are tertiary or quaternary carnivores, they receive ~10% of the energy from the previous trophic level.
• Because energy decreases sharply at each level, ecosystems rarely have more than 3–5 trophic levels.

Question 12.
Write important features of a sedimentary cycle in an ecosystem.
Answer:
In a sedimentary cycle, the materials involved in circulation between the biotic and abiotic components of the ecosystem are mostly non-gaseous, and the lithosphere (soil and rocks) acts as the main reservoir.

Key Features:

1. Non-gaseous Nature: Elements like phosphorus, calcium, and magnesium circulate in non-gaseous forms. Sulphur is an exception, as it has both sedimentary and gaseous phases.
2. Lithospheric Reservoir: The primary storage of nutrients is in rocks and soil, rather than in the atmosphere.
3. Slow Cycle: Sedimentary cycles are slower than gaseous cycles because nutrient release depends on weathering and soil processes.
4. Less Perfect Cycle: Compared to gaseous cycles, sedimentary cycles are less complete and often localized.

Question 13.
Outline salient features of carbon cycling in an ecosystem.
Answer:
Carbon is the most abundant element as it occurs in every organic substance. It forms 49% of the dry weight of organic matter. Carbon occurs as free carbon dioxide in the atmosphere and dissolved as CO₂ in the hydrosphere. Carbon dioxide is being added to the cycling pool of the atmosphere and hydrosphere through two processes, biological and non-biological.